1. Field of the Invention
This invention relates to a method of preparing a magnetic coating composition for a magnetic recording medium. This invention particularly relates to a method of preparing a magnetic coating composition by use of at least a two-shaft continuous kneading and mixing machine.
2. Description of the Prior Art
In the production of a magnetic recording medium, in order to quickly obtain a magnetic coating composition having a high dispersing quality, there has heretofore been known to carry out a series of processes for kneading ferromagnetic grains and a small amount of a binder solution by use of an incorporating machine such as a kneader exhibiting high shearing force before the ferromagnetic grains are dispersed in the binder solution by use of a dispersing machine such as a ball mill or a sand grinder. Also, in order to obtain a uniform paste by the kneading, it has heretofore been known to increase the kneading effect by use of a dispersant and/or a stabilizer or by use of a binder within a specified range of concentration. The techniques with respect to these preparation methods are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 46(1971)-3030, 48(1973)-104505, 49(1974)-14537, 53(1978)-76012, 55(1980)-25406, and 57(1982)-141026.
As for the preparation apparatus for carrying out the kneading and dispersion and the operating conditions of the apparatus in the preparation of the magnetic coating composition, it has heretofore been known to improve the dispersing quality by operating a kneader under specific shearing force or operating a dispersing machine or members in the dispersing machine under specific conditions.
In general, with the conventional methods of preparing a magnetic coating composition, ferromagnetic grains are kneaded together with a binder and an organic solvent, and the kneaded mixture thus obtained is diluted with a solution of a binder in an organic solvent, thereby to form the coating composition. However, with the medium dispersing machine such as a sand grinder or a ball mill used conventionally, dispersion cannot be achieved substantially. Therefore, with the conventional methods wherein the coating composition is prepared by carrying out the kneading and then the dispersion, the ferromagnetic grains cannot be substantially dispersed in a fine-grain form in the coating composition, and a re-dispersing process must be carried out after the dispersion. Accordingly, a long time has heretofore been taken for dispersion processing.
Also, with the conventionally used, batch type kneading machine such as a kneader or a roll mill, it is difficult to continuously achieve material feed, kneading and discharging. Furthermore, as the operating conditions of the kneading machine are defined in terms of the rotation speed and torque, it is difficult to control the quality of the kneaded mixture, and the quality fluctuates largely.
In order to eliminate the aforesaid problems, a novel method has been proposed in Japanese Unexamined Patent Publication No. 62(1987)-41274. The proposed method comprises the steps of, in the preparation of a magnetic coating composition, utilizing a continuous kneading and mixing machine for the kneading of materials such as ferromagnetic grains and a binder, and diluting the thus obtained kneaded mixture by use of a high-speed dispersion stirrer (flow jet mixer). However, it was found that the viscosity of the kneaded mixture obtained by the kneading with the continuous kneading and mixing machine often increases up to a value within the range of several millions of poises to several tens of millions of poises, and such a high-viscosity kneaded mixture cannot readily be diluted under high shearing force even though the dilution and dispersion are carried out in the manner as mentioned above. Therefore, small lumps of the high-viscosity kneaded mixture readily remain in the diluted mixture, and a uniform diluted mixture cannot readily be obtained.
As mentioned above, in the case where the continuous wet-type medium dispersing machine is used for further finely dispersing the diluted mixture, the dispersion processing time becomes long, and the degree of dispersion is not sufficiently high. Therefore, a magnetic recording medium made by use of the magnetic coating composition obtained in this manner cannot exhibit substantially high electromagnetic conversion characteristics.